Flat panel displays, such as liquid crystal displays (LCD), are necessary components of many kinds of electronic equipment, not least portable computers, mobile communication terminals etc. Such devices are by necessity powered by batteries and hence it is of vital importance that the energy in the batteries is used in a most effective manner.
To achieve high total energy efficiency in illumination systems for back lit or front lit LCD's, losses associated with the generation of polarized light should be minimized. A recent approach is to recycle light of undesired polarization instead of absorbing it. This can be done using the reflective polarizing foils that have recently become available from different manufacturers. Such foils directly transmit one polarization direction of light and reflect the undesired polarization back into the illumination system where it is recycled. Also polarizing foils have been proposed in which the undesired polarization direction of light is back scattered instead off reflected and subsequently recycled.
An alternative approach is to design a back light or a front light system that directly emits one polarization state of light. In a back light system, reflective or scattering polarization foils such as those described above are incorporated.
Another concept for polarization separation has also been used, such as polarization-dependent total internal reflection (TIR), oriented liquid crystalline coatings or combinations thereof. An example of the latter concept is presented in U.S. Pat. No. 5,729,311 in which is shown an illumination system for a flat panel picture display device.
The illumination system of U.S. Pat. No. 5,729,311 includes an optical waveguide into which light is coupled via an end face of the waveguide. The waveguide is provided with recesses that are filled with a material that is different from that of the waveguide. One of the materials is optically isotropic having an index of refraction np and the other material is optically anisotropic having indices of refraction no and ne. For the refractive indices it should hold that no or ne is equal or substantially equal to np in order that there is separation of polarization at the interface between isotropic and anisotropic material, resulting in a polarised emission.
A drawback of the device shown in U.S. Pat. No. 5,729,311 is that the recesses are located in the optical waveguide itself. This makes the waveguide more or less complicated, and hence relatively expensive, to manufacture.